In this project we utilize the fact that the sensitivity of a two-electrode bioimpedance measurement will be proportional to the square of the current density. Hence, using a needle tip as one of the electrodes in a two- or three-electrode system, will enable us to measure only on the properties of the tissue in the close vicinity of the tip, where the current density is very high.
The figure shows a 3 kHz impedance measurement as function of distance through a porcine tissue model (Modulus |Z| solid line and Phase angle φ dashed line). The typical high modulus and relatively small phase angle in fat can easily be differentiated from the lower modulus and larger phase angle in muscle. The green needle indicates the starting point of the horizontal needle insertion path.
A large number of various needle types are in use in clinics today. Typical fields of applications are blood sampling, ablation, biopsy, recording of bioelectrical signals, and administration of drugs. If we extend the definition of needles to include other small or sharp objects for hypodermic placement, e.g. cannulas and small surgical tools, the number of interventions using these types of objects becomes very large.
Experimental data published over the years show characteristic differences in electrical properties between tissue types. In this project we have used the characteristic impedance properties of common tissues to develop a guidance application for needles and other small types of clinical equipment.
In principle an impedance measurement requires only moderate electronic circuitry and is often convenient for implementation in relatively small, low cost applications. By placing small electrodes on the tip of a needle or other equipment, a real time monitoring of the tissue type proximal to the tip is enabled. The feasibility for miniaturization makes this method applicable for almost any type of clinical intervention.
More details can be found in the PhD thesis Needle Guidance in Clinical Applications based on Electrical Impedance